Rocket engine with one-piece combustion chamber step structure, and its fabrication

ABSTRACT

A rocket engine is prepared by fabricating a combustion chamber having an annular wall as a single piece of material. The wall has a first axial region with a first inner diameter, a second axial region with a second inner diameter greater than the first inner diameter, and an inner wall step transition between the first axial region and the second axial region. The wall is formed by furnishing an oversize blank of the wall material and electrical discharge machining or grinding it to the final size. The combustion chamber is attached to an injector by bonding an annular metallic deposit to the first axial region of the combustion chamber, providing an annular adaptor ring, first welding the adaptor ring to the metallic deposit, and second welding the adaptor ring to the injector.

BACKGROUND OF THE INVENTION

This invention relates to the structure of a liquid-fueled rocket engine, and, more particularly, to the structure of the combustion chamber and its manufacture, and the joining of the propellant injector and the combustion chamber.

A typical liquid-fueled rocket engine includes a cylindrically symmetric combustion chamber, with an injector attached to its injector end and a flared nozzle attached to its nozzle end. A liquid propellant including fuel and an oxidizer flows through injector ports in the injector and into the combustion chamber. The propellant is ignited in the combustion chamber. The hot gas resulting from the combustion expands through the nozzle and drives the rocket engine and the attached rocket structure in the direction opposite to that in which the nozzle is pointed.

The combustion chamber is generally cylindrical, but it is known to provide an outward step in the inner wall of the combustion chamber. That is, near the injector the inner surface of the wall of the combustion chamber has a first diameter, but at a location a portion of the distance from the injector to the nozzle there is an abrupt outward step in the inner surface of the wall to a second, greater diameter. The presence of this step promotes improved mixing and combustion of the fuel and the oxidizer, leading to improved performance of the rocket engine.

The inwardly facing surface of the wall of the combustion chamber is exposed to high temperature combustion gas during service. The wall is preferably made of a refractory material such as rhenium coated with iridium on the inwardly facing surface. The injector plate is much cooler, and is typically made of titanium. When the rocket engine is fired, there is a large temperature increase from room temperature and a large temperature gradient between the upper end of the combustion chamber and the adjacent injector, through the region where the two are attached.

It is conventional practice to attach the injector plate to the combustion chamber with a flange-and-bolt system or by welding. The flange-and-bolt system has the disadvantage that the mechanical seal is not well suited to withstand, without leaking, the high temperatures, large temperature change between room temperature and the service temperature, and large temperature gradients experienced during service. Welding of the extremely dissimilar metals is difficult, and in particular it is quite difficult to achieve reliable welds of rhenium to titanium with available welding technology. The high temperatures on the combustion chamber side of the joint and the high temperature gradient through the joint can lead to a premature failure of the joint and a shortening of the life of the engine. Even a small pinhole in the joint can be disastrous, as it results in a back leak of hot combustion gases from the interior of the combustion chamber.

Recent advances in the design of the rocket engine to allow higher temperature combustion and the use of more powerful propellants have resulted in even greater temperatures and temperature gradients. The existing stepped combustion chamber and combustion chamber/injector structures may be insufficient for operation in this environment. There is therefore a need for an improved approach to the manufacturing and structure of the combustion chamber, and for the attachment of the injector to the combustion chamber. The present invention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a rocket engine with an improved stepped combustion chamber structure and an approach for its manufacture. Also provided is an improved attachment between the injector and the combustion chamber. The rocket engine may be operated reliably at higher temperatures than possible with prior attachment procedures.

In accordance with the invention, a method of fabricating a rocket engine comprises the steps of fabricating a combustion chamber comprising an annular wall with a chamber wall inner surface and a chamber wall outer surface. The annular wall includes a first axial region having a first inner diameter of the chamber wall inner surface, and a second axial region having a second inner diameter of the chamber wall inner surface. The second inner diameter is greater than the first inner diameter. There is an inner wall step transition between the first axial region and the second axial region. The fabrication of the annular wall includes the steps of providing a combustion chamber wall blank that is oversize relative to the final combustion chamber wall, and removing material from the combustion chamber wall blank to reach the final annular wall. The step of removing is accomplished by a technique which does not substantially cold work the metal, such as electrical discharge machining or grinding. The fabrication of the rocket engine further includes providing an injector; and attaching the combustion chamber and the injector by bonding an annular metallic deposit to the first axial region of the combustion chamber, providing an annular adaptor ring, first welding the adaptor ring to the metallic deposit, and joining the adaptor ring to the injector.

The combustion chamber is preferably formed of a single piece of material, eliminating the need for some elements of detail structure required in other forms of the combustion chamber. This unitary construction is distinct from an alternative design for a step structure wherein the chamber wall is cylindrical of a constant diameter and the step is formed by a step collar insert within the combustion chamber. The present combustion chamber wall is manufactured from an oversize blank by removal of metal using a technique which does not unduly deform and cold/hot work the metal of the wall. The preferred approaches are metal removal by grinding and/or electrical discharge machining. In a preferred construction, the chamber wall is made of rhenium. At least a portion of the chamber wall inner surface may be coated with a layer of a material that provides enhanced resistance to the highly corrosive and erosive combustion gases. Such a coating material may be, for example, iridium or a ceramic.

The combustion chamber is joined to the injector by an attachment. The attachment preferably includes the annular metallic deposit bonded to the first axial region of the combustion chamber, the annular adaptor ring, and welds between the adaptor ring and the metallic deposit, and between the adaptor ring and the injector. The metallic deposit is preferably columbium, which is also known as niobium. (As used herein, a metal identified generically includes both the unalloyed metal and its alloys containing at least about 50 percent by weight of the pure metal. Thus, for example, as used herein unless otherwise stated, "columbium" includes both unalloyed columbium and its alloys.) The metallic deposit is formed and bonded to the combustion chamber wall by any operable technique, such as chemical vapor deposition, inertial welding, explosive cladding, or plasma spraying. Once this metallic deposit is in place, the annular adaptor ring is welded to the metallic deposit at one end, and joined to the injector at the other end. The axial location of the combustion chamber is established, in part, by the length of the adaptor ring, which allows the combustion chamber to be positioned for optimal performance. This axial positioning of the combustion chamber also allows the location of the bond joint between the metallic deposit and the combustion chamber to be positioned so as to maximize its performance, which typically is associated with minimizing differential thermal expansion and shock effects at the joint.

The present invention thus provides a rocket engine which has good efficiency of operation and specific impulse due to the presence of the internal step within the combustion chamber. It also has good reliability because of the integral construction of the combustion chamber, and the ability to position the bond joint between the metallic deposit and the combustion chamber at an optimal axial location. The rocket engine is readily manufactured by available techniques. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a rocket engine;

FIG. 2 is an enlarged detail of FIG. 1 in region 2--2, showing the injector and injector end of the combustion chamber;

FIG. 3 is a second embodiment of the enlarged detail of FIG. 2;

FIG. 4 is a block flow diagram of a method for constructing the rocket engine; and

FIG. 5 is a schematic sectional view of a metal blank from which the combustion chamber wall is fabricated.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a rocket engine 20, and FIGS. 2-3 show details of the rocket engine. The rocket engine 20 includes a combustion chamber 22 comprising a generally cylindrically symmetric annular wall 24 with a cylindrical axis 26, and an expansion nozzle 28. A narrowed throat region 29 is present within the expansion nozzle 28. The wall 24 and the expansion nozzle 28 may be fabricated separately and joined together, or they may instead be formed integrally. The cylindrical wall 24 has an outer cylindrical surface 30 and an inner cylindrical surface 32. The combustion chamber 22 has an injector end 34 and a nozzle end 36.

An injector 38 is attached to the injector end 34 of the combustion chamber 22. The injector 38 may be of any operable design, but is typically a plate having a plurality of injector ports 40 therein, of which only two are shown in FIG. 1. The propellant is provided to the combustion chamber 22 through the injector ports 40. Some of the injector ports 40a are supplied with a fuel through a fuel valve 42, and others of the injector ports 40b are supplied with an oxidizer through an oxidizer valve 44. The fuel and oxidizer flow through their respective injector ports 40 into the interior of the combustion chamber 22 and mix together. Where the mixture is hypergolic, as in the case of the preferred monomethyl hydrazine (fuel) and nitrogen tetraoxide/3 percent nitric acid (oxidizer), the mixture ignites spontaneously. In other cases where the mixture does not spontaneously ignite, an ignitor (not shown) would be provided. The gaseous combustion products of the combustion expand rearwardly and outwardly through the nozzle 28 and drive the rocket engine 20, and the spacecraft to which it is attached, in the opposite direction.

The wall 24 of the combustion chamber 22 includes two regions shown in FIGS. 2-3, a first axial region 24a and a second axial region 24b. The first axial region 24a is closest to the injector 38, and the second axial region 24b is further from the injector 38 along the cylindrical axis 26 and closer to the expansion nozzle 28. The wall 24a of the first axial region 24a has a first inner diameter 46 and a first outer diameter 48. The wall 24b of the second axial region 24 has a second inner diameter 50 and a second outer diameter 52. The second inner diameter 50 is larger than the first inner diameter 46. The second outer diameter 52 may be larger than the first outer diameter 48, as in FIG. 2, or not.

An abrupt, outwardly extending inner wall step transition 54 separates the first axial region 24a from the second axial transition 24b. The step transition 54 promotes thorough mixing and combustion of the fuel and the oxidizer introduced into the combustion chamber 22 through the injector 38. Preferably but not necessarily, the entire wall 24 of the combustion chamber 22 is a single, integral metallic piece, as distinct from other embodiments wherein the wall 24 and the step structure might be formed as two or more metallic pieces that are joined together. By making the entire wall 24 of a single material, there is no differential thermal expansion in the wall between different materials, which could lead to thermal expansion strains and thermal shock. The absence of these effects improves the reliability of the wall and hence the engine.

An attachment 56 joins the combustion chamber 22 to the injector 38. The attachment 56 includes several elements and several joints. The attachment 56 must mechanically join the combustion chamber 22 and the injector 38 to bear the loads imposed during handling and during the large temperature changes and gradients which are experienced during service, and also provide a seal against the leakage of hot gas at the injector end 34 of the combustion chamber 22.

The attachment 56 includes an annular metallic deposit 58 bonded in a metal-to-metal contact 59 to the first axial region 24a of the combustion chamber 22, preferably to an outer surface 30a of the first axial region 24a, at the injector end 34. The metallic deposit 58 is preferably about 0.110 inch thick (in the radial direction perpendicular to the cylindrical axis 26) at its thickest location and tapers toward the wall 24 with increasing distance from the injector 38.

The attachment 56 further includes an annular adaptor ring 60 joined to the metallic deposit 58 by a joint 62, which is most preferably a welded joint. The length of the adaptor ring 60 in the axial direction parallel to the axis 26 may be selected to position the metal-to-metal contact 59 of the metallic deposit 58 to the combustion chamber 22 at any desired axial location. FIG. 2 illustrates a relatively longer adaptor ring 60 that positions the contact 59 further from the injector 38, and FIG. 3 illustrates a relatively shorter adaptor ring 60 that positions the contact 59 closer to the injector 38. The axial location of the contact 59 may be important to the proper functioning of the engine 20, because the contact 59 is typically between dissimilar metals with different coefficients of thermal expansion, as will be described subsequently. The region of this contact 59 is therefore potentially susceptible to damage during testing and service due to differential thermal expansion and thermal shock effects. The use of the adaptor ring 60 of selectable length allows the designer of the engine to position the region of the contact 59 at an axial location where the potential damage to the engine will be minimal. Once the axial position of the contact 59 is established in this manner, the axial position of the step transition 54 is determined according to the length of the first axial region 24a. The exact axial positions of the contact 59 and the step transition 54 are selected according to the nature of the materials used to form the engine, the type of engine, the size of the engine, and the mission of the engine. The selection of these axial positions is not a part of the present invention, which instead provides engine designers the capability to establish the locations of the key elements of the engine.

The opposite end of the adaptor ring 60 is joined by a joint 64 to the injector 38. The joint 64 may be of any operable type. FIG. 2 illustrates a preferred embodiment wherein the joint 64a is a weld joint, and FIG. 3 illustrates an alternative embodiment wherein the joint 64b is a bolted flange joint.

In a preferred embodiment, the wall 24 of the combustion chamber 22 is made of rhenium, most preferably unalloyed rhenium, with an optional 0.003-0.005 inch thick coating 66 on the inner cylindrical surface 32. (As used herein, a metal identified generically includes both the unalloyed metal and its alloys containing at least about 50 percent by weight of the pure metal. Thus, for example, "rhenium" includes both pure rhenium and its alloys.) The coating 66 is a material which withstands the erosion and corrosion of the hot combustion gas of the engine during service more effectively than does the material of the wall 24. Preferably, the coating 66, where used, is iridium or a ceramic. The coating 66 is applied by any operable technique for the selected coating material. For the preferred iridium coating, the coating 66 is preferably applied by plasma spraying, sputtering, or chemical vapor deposition. Areas that are not to be coated are masked during the coating operation. The metallic deposit 58 is preferably made of unalloyed columbium or C103 alloy having a composition of 10 weight percent hafnium, 1 weight percent titanium, balance columbium. The adaptor ring 60 is preferably made of columbium, most preferably C103 alloy having a composition of 10 weight percent hafnium, 1 weight percent titanium, balance columbium. The injector 38 is preferably made of titanium, most preferably an alloy of 6 weight percent aluminum, 4 weight percent vanadium, balance titanium. The titanium/columbium weld 64a between the injector 38 and the adaptor ring 60, and the columbium/columbium weld 62 between the adaptor ring 60 and the metallic deposit 58, are both readily accomplished by electron beam welding. Alternatively, as shown in FIG. 3, the joint 64a may be another type of joint such as a bolted joint.

The metallic deposit 58 joins the adaptor ring 60 to the wall 24 of the combustion chamber 22. The columbium adaptor ring 60 does not readily braze or weld directly to the relatively thin rhenium wall 24 of the combustion chamber 22 with the required strength and soundness. The metallic deposit 58 is first deposited onto the wall 24 with a metallurgical bond, and then the adaptor ring 60 is welded to the deposit 58.

FIG. 4 illustrates the preferred approach to the fabrication of the rocket engine 20. The combustion chamber wall 24 is first fabricated, numeral 80. The wall 24 is preferably fabricated as a single piece of metal, with the step transition 54 formed integrally. The wall 24 is fabricated from a single piece of starting material, termed a "blank" 90, shown in FIG. 5. The blank 90 is prepared by any technique suitable for the material of the wall 24. In the case of the preferred rhenium, the blank 90 may be prepared, for example, as a thick-walled cylinder by consolidation of metal powders, casting, or forming. The blank 90 is oversize in wall thickness relative to the final thickness of the wall 24, so that material must be removed from the blank 90 to reach the final configuration of the combustion chamber wall 24. The selection of the location and amount of material to be removed 92 depends on the precise desired shape and dimensions of the wall 24. In general, metal 92a may be removed from the inner diameter, and/or metal 92b may be removed from the outer diameter. The step transition 54 is fabricated in place by this metal-removal technique.

The metal to be removed is removed by a technique which does not substantially mechanically deform or cold work the remaining metal that ultimately forms the wall 24. The preferred techniques to achieve the metal removal are grinding and electrical discharge machining (EDM), both known techniques for other applications. In grinding, metal is removed with an abrasive action, and in EDM, metal is removed by an electrical action. In each case, the underlying remaining metal is not unduly deformed or cold worked. Such deformation and/or cold working of the remaining metal is deleterious to the performance of the wall 24 in service within the rocket engine, because it produces a brittle cold-worked microstructure. The grinding and EDM are distinct from other metal removal techniques such as removal with a sharpened machine tool such as a cutting tool, where the underlying metal is typically cold worked, leading to the embrittled structure. Machining techniques which produce substantial deformation to the underlying structure are not acceptable for use herein. Grinding and EDM are slower metal removal techniques than machining with a cutting tool, but the improved final performance of the article warrants the extra time and expense.

The material of the metallic deposit 58 is provided and bonded to the wall 24 of the combustion chamber 22, numeral 82, to form the metal-to-metal contact 59. Any operable bonding technique may be used. Welding and brazing are preferably not used, because it is difficult obtain a sound bond and contact to rhenium by these techniques. Instead, the preferred technique is to deposit the metallic deposit 58 onto the wall 24, thereby establishing the metal-to-metal bond. The preferred deposition technique is chemical vapor deposition. Other operable techniques include, but are not limited, to, inertial welding, explosive cladding, and plasma spraying.

The injector 38 is provided, numeral 84, and the adaptor ring 60 is provided, numeral 86. The components are joined together, numeral 88. The adaptor ring 60 is joined to the metallic deposit 58 by any operable technique at the joint 62. The preferred approach is welding, most preferably electron beam welding, because the joint 62 is in or close to the hot zone of the combustion chamber. The adaptor ring 60 is joined to the injector by any operable technique at the joint 64. Because the joint 64 is at a lower temperature than the joint 62 during service, other types of joining techniques are operable. Welding by electron beam welding is preferred, as shown in FIG. 2, or, for example, a flange/bolt structure may be used, as shown in FIG. 3.

To complete the fabrication, the expansion nozzle 28 is attached to the combustion chamber 22, if the expansion nozzle was prepared as a separate component from the combustion chamber 22. The propellant lines are connected, and any other elements of the rocket engine, such as instrumentation probes, are attached.

Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims. 

What is claimed is:
 1. A method of fabricating a rocket engine, comprising the steps offabricating a combustion chamber comprising an annular wall with a chamber wall inner surface and a chamber wall outer surface, the annular wall includinga first axial region having a first inner diameter of the chamber wall inner surface, a second axial region having a second inner diameter of the chamber wall inner surface, wherein the second inner diameter is greater than the first inner diameter, and an inner wall step transition between the first axial region and the second axial region, the step of fabricating including the steps of providing a combustion chamber wall blank that is oversize relative to the final combustion chamber wall, and removing material from the combustion chamber wall blank to reach the final annular wall by a removal method which does not substantially deform and cold work any material which is not removed from the wall blank; providing an injector; and attaching the combustion chamber and the injector, the step of attaching including the steps ofbonding an annular metallic deposit to the first axial region of the combustion chamber, providing an annular adaptor ring, first welding the adaptor ring to the metallic deposit, and second welding the adaptor ring to the injector.
 2. The method of claim 1, wherein the step of fabricating a combustion chamber includes the step of removing material by electrical discharge machining.
 3. The method of claim 1, wherein the step of fabricating a combustion chamber includes the step ofremoving material by grinding.
 4. The method of claim 1, wherein the step of fabricating a combustion chamber includes the step offabricating the combustion chamber from rhenium.
 5. The method of claim 1, wherein the step of fabricating a combustion chamber includes the step ofapplying a coating to at least a portion of the chamber wall inner surface.
 6. The method of claim 5, wherein the step of applying a coating includes the step ofapplying a coating selected from the group consisting of iridium and a ceramic.
 7. The method of claim 1, wherein the step of fabricating the combustion chamber includes the step offabricating the combustion chamber as a single piece of material.
 8. The method of claim 1, wherein the step of bonding an annular metallic deposit includes the step ofbonding the metallic deposit to the first axial region by a method selected from the group consisting of chemical vapor deposition, inertial welding, explosive forming, and plasma spraying.
 9. The method of claim 1, wherein the step of bonding an annular metallic deposit includes the step ofproviding a metallic deposit material of columbium.
 10. The method of claim 1, wherein the step of fabricating a combustion chamber includes the steps ofproviding the first axial region having a first outer diameter of the chamber wall outer surface, providing the second axial region having a second outer diameter of the chamber wall outer surface, wherein the second outer diameter is greater than the first outer diameter, and providing an outer wall step transition between the first axial region and the second axial region. 